1. Field of the Invention
The present invention relates to a control apparatus, a control method and an engine control unit which apply a controlled object with a control input calculated in accordance with a deviation of an output of the controlled object from a target value to converge the output of the controlled object to the target value, and more particularly, to a control apparatus, a control method and an engine control unit of this kind which are configured to calculate a control input by switching from calculation processing based on one of a Δ modulation algorithm, a ΔΣ modulation algorithm and a ΣΔ modulation algorithm, to calculation processing based on a response specified control algorithm, and vice versa.
2. Description of the Prior Art
The applicant has previously proposed a control apparatus of the type mentioned above for controlling an air/fuel ratio of an air/fuel mixture for an internal combustion engine, for example, in Japanese Patent Application No. 2001-400988. This control apparatus comprises an oxygen concentration sensor disposed downstream of a catalyzer in an exhaust passage of the internal combustion engine, an ADSM controller for controlling the air/fuel ratio of an air/fuel mixture in accordance with a control algorithm based on a ΔΣ modulation algorithm, and a PRISM controller for controlling the air/fuel ratio of the air/fuel mixture in accordance with a control algorithm based on a sliding mode control algorithm.
This control apparatus executes the air/fuel ratio control using the ADSM controller and PRISM controller, one of which is selected in accordance with a particular operating condition of the internal combustion engine. More specifically, the ADSM controller relies on the control algorithm based on the ΔΣ modulation algorithm to calculate a target air/fuel ratio in accordance with a deviation of an output of the oxygen concentration sensor from a predetermined target value for converging the output of the oxygen concentration sensor to the target value, and controls the air/fuel ratio of the air/fuel mixture in accordance with the target air/fuel ratio thus calculated. The PRISM controller in turn relies on the control algorithm based on the sliding mode control algorithm to calculate a target air/fuel ratio in accordance with a deviation of the output of the oxygen concentration sensor from a predetermined target value for converging the output of the oxygen concentration sensor to the target value, and controls the air/fuel ratio of the air/fuel mixture in accordance with the target air/fuel ratio thus calculated.
With the air/fuel ratio control conducted by the ADSM controller or PRISM controller, the output of the oxygen concentration sensor is controlled to converge to the target value, thereby maintaining a high exhaust gas purification percentage of the catalyzer in consequence. In this event, with the air/fuel ratio control conducted by the ADSM controller, when the output of the oxygen concentration sensor is richer than the target value (i.e., exhaust gases supplied to the catalyzer is richer), the ADSM controller controls the air/fuel ratio of the air/fuel mixture such that the output of the oxygen concentration sensor converges to the target value at a lower rate than when the output of the oxygen concentration sensor is leaner than the target value. When the air/fuel ratio of the air/fuel mixture is controlled to rapidly converge the output of the oxygen concentration sensor, which is richer than the target value, to the target value, the target air/fuel ratio is set to a leaner value, thereby supplying the catalyzer with exhaust gases the air/fuel ratio which is made rapidly leaner. As a result, the NOx purification percentage is degraded due to an extremely lean catalyst in an upstream region of the catalyzer. The aforementioned control strategy is taken for preventing such degradation of the NOx purification percentage.
According to the conventional air/fuel ratio control apparatus described above, in the event of switching from the air/fuel ratio control conducted by the ADSM controller to the air/fuel ratio control conducted by the PRISM controller, if the output of the oxygen concentration sensor has been much richer than the target value before the switching, the PRISM controller sets the target air/fuel ratio as the control input to a leaner value than the value calculated by the ADSM controller after the switching of the air/fuel ratio control with the intention that the output of the oxygen concentration sensor converges to the target value at a rate higher than the ADSM controller. This setting can cause a sudden and large change of the air/fuel ratio of the air/fuel mixture toward the lean side, resulting in a step before and after the switching. As a result, the NOx purification percentage can be degraded due to the excessively lean catalyst in the upstream region of the catalyzer. This is because the PRISM controller employs the sliding mode control algorithm, which is one type of response specified control algorithm, to converge the output of the oxygen concentration sensor to the target value at a higher rate than the ADSM controller, thereby improving the accuracy of the air/fuel ratio control.